Hi All,
Andy, I was pondering about the extra $90 US or so to upgrade from the Hammond to the Lundahl and if there was appreciable gain in sound quality, but I might just wait and get the Lundahl

Nicoch, I was considering getting some CMC’s wound, but I had a look in the shed and noticed I had some semi vintage A&R Australian made 1.5H 4 ohm 500mA chokes. If run in parallel in the + leg (750mH 1A) they may be useful in trying a passive LCL filament supply.

I ended up getting 400mV ripple at the end. Thomas is using a similar supply with 2 LL2733’s in series. I modelled his psu and it also yields the same ripple at the end. I used 10nF to simulate the 26’s filament. If I get some time this weekend I’ll try it out

But…

I have a question in regard to the potential damage to the filament if the filament supply is on without any B+, since filament bias applies approx 6 times the filament voltage. Forgive my ignorance but in order to get the correct bias voltage and plate current (6mA) one needs to apply filament (bias) voltage (9V) and B+ (150V)? Any advice to clarify this would be most appreciated.

I have a question in regard to the potential damage to the filament if the filament supply is on without any B+, since filament bias applies approx 6 times the filament voltage

Richard, when using filament bias, the resistor wired in series with the filament produces the voltage to bias the tube at your chosen point. There will never be 6 times the filament voltage applied to the filament. I can't see any issues arising from running without B+.

Because the bias, plate load and the B+ you apply are fixed, the only variable is plate current, which will settle to a value consistent with the other fixed parameters (refer to datasheet plate curves see what it will be).

Don't rule out the 01A as an alternative with low filament current 250mA - would suit your choke. It's very, very good - only very slightly bettered by the 26. My rough calculations for filament bias for this was 105v on anode from a 105v glow tube. 3mA through the tube, 18 ohm cathode resistor at 1 watt and 4.5v on the cathode (top of resistor). I tried this out and was very impressed - sound was very lively and engaging, just not quite as subtle as the 26. Supply voltage about 9.5v.

Here is my experience with filament bias. It is pretty good and easy use method. Easiest I suppose. Gives an impressive boost to the sound than auto bias option. But the winner is grid bias for me. I have no doubts because I have tried all three methods in my 26 pre and grid biasing gave me the best results so far. It gave more control of changing tube parameters and sound is exceptionally life like and accurate. Andy, I would suggest you to try this if you have not tried it yet. Do not know whether the cap in the signal path does anything to the sound but I heard most accurate sounds of instruments out of this method. Vocals are simply magical.

Also I have changed my filament supply from SMPS to a TRAFO+LM317. This added some more to the overall result. I had a filament PSU kit in hand (JIMS audio@Ebay) and it worked perfectly. I wonder why I didn't try it before . Slight humming sound that SMPS was giving is now completely gone and it is dead silent now.

BTW, I found a new use for my 50W resister. I used it as heat sink for my HVPSU (SALAS) IRF840 . It really worked well.

Hi All,
Andy, I was pondering about the extra $90 US or so to upgrade from the Hammond to the Lundahl and if there was appreciable gain in sound quality, but I might just wait and get the Lundahl

<snip>

Rich

Save your money for the Lundahls, they are in most cases better designed and better made than Hammonds. IMHO This should be audible and measurable. If cost is an over bearing consideration you might want to look at Electra-Prints offerings as well - I've generally found them better sounding than their Hammond equivalents in my very limited experience.

I have a question in regard to the potential damage to the filament if the filament supply is on without any B+, since filament bias applies approx 6 times the filament voltage. Forgive my ignorance but in order to get the correct bias voltage and plate current (6mA) one needs to apply filament (bias) voltage (9V) and B+ (150V)? Any advice to clarify this would be most appreciated.

Rich

No problem applying filament voltage and bias prior to plate voltage, what you need to avoid is applying plate voltage without bias being present however derived.

Conventional cathode bias is safe too as bias will be developed almost immediately when plate current starts to flow.

The filament derived bias discussed here is perfectly fine IMHO and sonically far superior to conventional cathode bias, that said the idea of 9W of power in a resistor to create the bias for a 6mA operating point seems well.. inefficient..

Note that you should try to operate the 26 filament at as close to 1.05A/1.5V as reaonably possible no less than -10% is ok, but anything less and the transconductance starts to drop pretty drastically and in my long (10+ yr) experience with this tube both compromises tube life and most directly sound quality.

I much prefer grid bias with the 26/12/01 and a very high quality coupling cap. (I use Vcaps, but the Rel TFT is also quite good) 9V alkaline batteries can provide great service in this application and last many years. Do not use a rechargeable battery for grid bias!

My original 26 pre-amp article is still available at Positive Feedback Online, should be some insight for those interested in such things. (The design is now actually over 10 years old, although the article is more recent. More current versions of that design have been well received in Asia. Coolzero and I have chatted extensively about the 26 over the last year or so..)